The polymerisation of olefins in gas phase fluidised bed reactors is well-known. Control of such processes is required to ensure stable fluidisation and stable production of polymer with desired characteristics. One particular problem which can occur is the formation of agglomerates. As well as control during steady-state it is also important to control the process during periods of non-steady-state operation, such as during start-ups.
At non-steady-state conditions it can be especially difficult to avoid the presence of agglomerates because it is especially difficult to control the density and melt index of polymer it is desired to produce.
A number of applications have attempted to address such problems during start-ups and at steady state.
WO 99/31143, for example, relates to a process for starting-up a polymerisation process in a gas phase fluidised bed which uses a catalyst based on chromium oxide. In the process described in this document the start-up involves two separate stages, the first in which olefin is introduced so as to maintain a constant partial pressure in the reactor, and the second in which olefin is introduced at a constant flow rate.
WO 99/31142 also relates to a process for starting-up a polymerisation process in a gas phase fluidised bed, but this time using a Ziegler-Natta type catalyst. In the process described in this document the start-up involves increases in the partial pressure of olefin and the rate of introduction of catalyst, whilst maintaining constant the ratios of the partial pressures of the olefin to hydrogen and to any comonomers.
WO 02/51027 relates to a start-up procedure for processes using at least two different catalyst compounds. The procedure involves calculating a trajectory, from elements including catalyst deactivation rate constants (kd), for the rate of introduction of a catalyst system, into a reactor, and introducing the olefin monomer, a catalyst system, optional comonomer, and optional chain transfer or termination agents into a polymerization reactor in a manner such that the catalyst system introduction rate is manipulated to follow the trajectory until a desired production rate is achieved. WO02/51027 teaches to keep constant throughout start-up and polymerisation the ratios of comonomer and hydrogen to ethylene.
WO2005007711 relates to a process for the polymerisation or copolymerisation in the gas phase of ethylene, which process comprises a pre start-up operation characterized in that, prior to the introduction of the catalytic system in the reactor, the start-up bed is heated at a high temperature.
U.S. Pat. No. 5,534,607 and U.S. Pat. No. 5,306,792 relate to operation of processes operating at steady state. In particular, U.S. Pat. No. 5,534,607 relates to a continuous process for polymerisation of an alpha-olefin using a Ziegler-Natta type catalyst in which the reactor is fed with alpha-olefin at a constant rate and U.S. Pat. No. 5,306,792 relates to a continuous process for polymerisation of an alpha-olefin using a catalyst based on chromium oxide in which the reactor is fed with alpha-olefin and catalyst at constant rates.